505 research outputs found
A semiconductor source of triggered entangled photon pairs?
The realisation of a triggered entangled photon source will be of great
importance in quantum information, including for quantum key distribution and
quantum computation. We show here that: 1) the source reported in ``A
semiconductor source of triggered entangled photon pairs''[1. Stevenson et al.,
Nature 439, 179 (2006)]} is not entangled; 2) the entanglement indicators used
in Ref. 1 are inappropriate, relying on assumptions invalidated by their own
data; and 3) even after simulating subtraction of the significant quantity of
background noise, their source has insignificant entanglement.Comment: 5 pages in pre-print format, 1 tabl
Diagnosis, prescription and prognosis of a Bell-state filter by quantum process tomography
Using a Hong-Ou-Mandel interferometer, we apply the techniques of quantum
process tomography to characterize errors and decoherence in a prototypical
two-photon operation, a singlet-state filter. The quantum process tomography
results indicate a large asymmetry in the process and also the required
operation to correct for this asymmetry. Finally, we quantify errors and
decoherence of the filtering operation after this modification.Comment: 4 pages, 4 figure
Quantum filter for non-local polarization properties of photonic qubits
We present an optical filter that transmits photon pairs only if they share
the same horizontal or vertical polarization, without decreasing the quantum
coherence between these two possibilities. Various applications for
entanglement manipulations and multi-photon qubits are discussed.Comment: 7 pages, including one figure, short discussion of error sources
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Properties of implanted and CVD incorporated nitrogen-vacancy centers: preferential charge state and preferential orientation
The combination of the long electron state spin coherence time and the optical coupling of the ground electronic states to an excited state manifold makes the nitrogen-vacancy (NV) center in diamond an attractive candidate for quantum information processing. To date the best spin and optical properties have been found in centers deep within the diamond crystal. For useful devices it will be necessary to engineer NVs with similar properties close to the diamond surface. We report on properties including charge state control and preferential orientation for near surface NVs formed either in CVD growth or through implantation and annealing
Sub-microsecond correlations in photoluminescence from InAs quantum dots
Photon correlation measurements reveal memory effects in the optical emission
of single InAs quantum dots with timescales from 10 to 800 ns. With above-band
optical excitation, a long-timescale negative correlation (antibunching) is
observed, while with quasi-resonant excitation, a positive correlation
(blinking) is observed. A simple model based on long-lived charged states is
presented that approximately explains the observed behavior, providing insight
into the excitation process. Such memory effects can limit the internal
efficiency of light emitters based on single quantum dots, and could also be
problematic for proposed quantum-computation schemes.Comment: 8 pages, 8 figure
On the indistinguishability of Raman photons
We provide a theoretical framework to study the effect of dephasing on the
quantum indistinguishability of single photons emitted from a coherently driven
cavity QED -system. We show that with a large excited-state detuning,
the photon indistinguishability can be drastically improved provided that the
fluctuation rate of the noise source affecting the excited state is fast
compared with the photon emission rate. In some cases a spectral filter is
required to realize this improvement, but the cost in efficiency can be made
small.Comment: 18 pages, 3 figures, final versio
Enabling single-mode behavior over large areas with photonic Dirac cones
Many of graphene's unique electronic properties emerge from its Dirac-like
electronic energy spectrum. Similarly, it is expected that a nanophotonic
system featuring Dirac dispersion will open a path to a number of important
research avenues. To date, however, all proposed realizations of a photonic
analog of graphene lack fully omnidirectional out-of-plane light confinement,
which has prevented creating truly realistic implementations of this class of
systems. Here we report on a novel route to achieve all-dielectric
three-dimensional photonic materials featuring Dirac-like dispersion in a
quasi-two-dimensional system. We further discuss how this finding could enable
a dramatic enhancement of the spontaneous emission coupling efficiency (the
\beta-factor) over large areas, defying the common wisdom that the \beta-factor
degrades rapidly as the size of the system increases. These results might
enable general new classes of large-area ultralow-threshold lasers,
single-photon sources, quantum information processing devices and energy
harvesting systems
Creating diamond color centers for quantum optical applications
Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state
qubits. This is because of their large dipole moment, convenient level
structure and very long room-temperature coherence times. In general, a
combination of ion irradiation and subsequent annealing is used to create the
centers, however for the rigorous demands of quantum computing all processes
need to be optimized, and decoherence due to the residual damage caused by the
implantation process itself must be mitigated. To that end we have studied
photoluminescence (PL) from NV, NV and GR1 centers formed by ion
implantation of 2MeV He ions over a wide range of fluences. The sample was
annealed at C to minimize residual vacancy diffusion, allowing for
the concurrent analysis of PL from NV centers and irradiation induced vacancies
(GR1). We find non-monotic PL intensities with increasing ion fluence,
monotonic increasing PL in NV/NV and GR1/(NV + NV) ratios, and
increasing inhomogeneous broadening of the zero-phonon lines with increasing
ion fluence. All these results shed important light on the optimal formation
conditions for NV qubits. We apply our findings to an off-resonant photonic
quantum memory scheme using vibronic sidebands
Production of oriented nitrogen-vacancy color centers in synthetic diamond
The negatively charged nitrogen-vacancy (NV-) center in diamond is an
attractive candidate for applications that range from magnetometry to quantum
information processing. Here we show that only a fraction of the nitrogen
(typically < 0.5 %) incorporated during homoepitaxial diamond growth by
Chemical Vapor Deposition (CVD) is in the form of undecorated NV- centers.
Furthermore, studies on CVD diamond grown on (110) oriented substrates show a
near 100% preferential orientation of NV- centers along only the [111] and
[-1-11] directions, rather than the four possible orientations. The results
indicate that NV centers grow in as units, as the diamond is deposited, rather
than by migration and association of their components. The NV unit of the NVH-
is similarly preferentially oriented, but it is not possible to determine
whether this defect was formed by H capture at a preferentially aligned NV
center or as a complete unit. Reducing the number of NV orientations from 4
orientations to 2 orientations should lead to increased optically-detected
magnetic resonance contrast and thus improved magnetic sensitivity in
ensemble-based magnetometry.Comment: 13 Pages (inlcuding suplementary information), 4 figure
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